Electronic imaging or scanning systems are commonly used to transform an image from one form, such as a paper original, to an electronic form, such as a digital or analog signal. Once an image is converted to electronic form, many uses of that signal become are possible, including, without limitation, copying of the image onto a piece of paper, projection of the image onto a video display terminal, transmitting the image to a remote location, and subjecting the image to further image processing, such as by a computer, an optical pattern recognition device, or the like.
Line imaging systems typically include a linear array of photosensitive elements, such as photosensors, as well as a light source operatively disposed to provide flood illumination of the surface being scanned. Then either the image on that surface is moved in a direction perpendicular to the longitudinal axis of the sensor array, or the sensor array is moved in a direction perpendicular to the longitudinal axis of that surface so as to scan a stationary image. Since the light reflected from the image-bearing surface varies depending upon the portion of the image being scanned, a darker portion of the image will cause the photosensitive elements to receive less light, while a brighter portion will cause the photosensitive elements to receive more light.
In practice, it has been determined that, due to the inevitable variations in light intensity as well as the variations in the photoresponsive characteristics of individual elements, the signals produced by the reflection of light from areas of the image which are equally bright can be unequal.
It is known in the prior art to compensate for both photosensitive element and illumination non-uniformities of a particular linear array by calibrating the light output data of each element in response to a uniform background image. This accumulated data produces a calibration curve which may be stored in a memory (such as a RAM or a ROM) and then used to compensate the signals sensed elements of the array which combine to form the imaging system. However, because of photosensitive element and light output variability, the calibration curve for a particular linear scanning array can change with time, thereby causing a degradation of the images produced and the need for constant recalibration.
Also known in the art is a scanning apparatus which is adapted to simultaneously sense both the light emanating directly from the source and the light reflected from the scanned image. For each photosensitive element in such an array, a value corresponsing to the peak light intensity experienced in scanning the surface is stored; this peak value is then subtracted from the intensity of light previously measured and compared to the reference value. Based upon this comparison, the measured intensity is determined to correspond to either a bright image point or a dark image point. However, such systems are unnecessarily complicated, and require complex electronics.
It is also known in the art to scan surface areas as large as a one meter by two meter copy board by having a movable, white flexible material such as polyester, stretched across and in front of a correspondingly large rigid surface. An image, such as characters or graphics are drawn or otherwise formed upon the surface of the flexible material using, for example black or dark colored erasable markers. Spaced rollers operatively disposed on either distal side of the copyboard are driven in synchronism by a pair of microprocessor controlled stepping motors so as to roll the flexible sheet onto one of the rollers. The image in such systems is scanned under the supervision of a microprocessor control by an array of one or more integrated CCD sensors positioned at a distance of about one meter from the take-up roller. A light source positioned adjacent to the take-up roller and an optical lens system positioned between the take-up roller and CCD array cooperate to project the image onto the CCD. In this manner, a CCD device which may be 25 millimeters or less in linear dimension is capable of scanning the entire surface of the flexible material as the material is being scrolled. Once digitized into electronic form, the image is sent by the microprocessor to a printer which then provides a hard copy of the scanned image. That hard copy generally has been greatly reduced in size compared to the original on the copyboard surface. Such systems typically exhibit fairly low resolution, on the order of 1.0 to 1.5 photosensors or pixels per millimeter or less of original copy, since higher resolution is normally not required for making reduced size copies.
Since the CCD photosensor array and lens system must be spaced relatively far from the linear strip of the image being scanned in order to allow light from the strip to be focused down to the size of the photosensor array, such systems are quite bulky (the copyboard must be thick enough to contain the optical elements). For the same reason, it would not be possible to mount the optical system and CCD sensor array on an elongated movable arm which moves across the stationary image-bearing surface to be copied. Also, since CCDs are quite expensive, it is uneconomical to provide a multitude of integrated CCD photosensitive elements (said photosensitive elements having about two orders of magnitude more resolution than needed) arranged in a large linear array on a movable arm in order to scan a large area image on a stationary board surface to produce a low resolution copy.
In light of the foregoing, it will be readily appreciated that there remains a need for an imaging system which includes a low cost linear array of photosensitive elements, which elements span an elongated linear distance such as one-half meter to one meter or more and which system can be utilized to scan and digitize images on large light-colored surfaces having one or more square meters of surface area. Furthermore, there remains a need for an imaging system including such a linear photosensitive array which provides for such larger image-bearing surfaces to be scanned quickly, efficiently, reliably and in a manner which automatically compensates for variations in image tone, differing individual photoresponsive element characteristics, and changing illumination conditions, such as from the aging (and hence deterioration) of the flood illumination light source.
There also remains a need for a large, low cost linear photosensitive array which features excellent signal-to-noise ratios and the ability to detect relatively light colors (such as low contrast red, light green and light blue) on a white or light colored background surface; as well as darker colors, such as black, brown, dark blue and dark red, on such a light colored background surface.
A further novel feature of the subject invention, which feature finds no response in any prior art, is the fabrication of the linear photoresponsive array on a flexible substrate. In this manner, the flexible photoresponsive array can be used in ways heretofore impossible so as to scan images and conditions on contoured surfaces.
It is desirable, therefore, to provide a simple, inexpensive line imaging system which is capable of compensation, on a substantially instantaneous basis, for variability and changes in illumination intensity as well as instability of photoresponsive element response.
These and many other advantages of the subject invention will become apparent from the drawings, the detailed description and the claims which follow.